1. Field of the Invention
The present invention relates to a data inverting circuit, and more particularly to a data inverting circuit for producing, from an analog input signal whose voltage is higher (or lower) than a constant DC voltage (hereinafter referred to as an "inverting reference voltage"), an output signal having a waveform that is of symmetrical relationship to the waveform of the input signal with respect to the inverting reference voltage as a hypothetical axis of symmetry.
2. Description of the Related Art
Data inverting circuits of the type described above are used, for example, to invert a voltage applied to a liquid crystal for preventing the liquid crystal from being charged in an analog full-color liquid crystal display unit. As shown in FIG. 1(a) of the accompanying drawings, the data inverting circuit produces, from high-frequency analog input signal VIN whose voltage is lower (or higher) than inverting reference voltage VREF, output signal VOUT having a waveform that is of symmetrical relationship to the waveform of input signal VIN with respect to inverting reference voltage VREF as a hypothetical axis of symmetry. The frequency of input signal VIN applied to the liquid crystal ranges from several tens of MHz to about hundred MHz. However, there is a demand for higher input signal frequencies as larger liquid crystal display units are desired.
Heretofore, a negative-feedback operational amplifier as shown in FIG. 1(b) of the accompanying drawings has been used as such a data inverting circuit. As shown in FIG. 1(b), DC inverting reference voltage VREF is applied from an external source to a noninverting input terminal of operational amplifier 33, whose inverting input terminal is supplied with analog input signal VIN from input terminal 1 through resistor R31. Resistor R32 having the same resistance as that of resistor R31 is connected as a negative-feedback loop between the output terminal and the inverting input terminal of operational amplifier 33. The data inverting circuit produces an output signal VOUT from output terminal 2 connected to the output terminal of operational amplifier 33.
With the conventional data inverting circuit, the phase of output signal VOUT may change to a large extent due to the parasitic capacitance of feedback resistor R32. Since it is difficult to maintain a sufficient phase margin, the data inverting circuit may oscillate due to a positive feedback loop. Usually, the data inverting circuit is prevented from oscillating by inserting a phase-compensating capacitor (not shown). However, the phase-compensating capacitor delays the response of operational amplifier 33, which is thus unable to respond to quick input signals.